Conventional static induction thyristors (hereinafter referred to as SI thyristor) formed in diode construction provided with a gate have self turn-off function by which the main current can be interrupted by the gate. However, the voltage drop in the ON condition (hereinafter called as ON voltage) is high and the voltage gain is low. Thus, there are several points to be improved in its characteristics. A method for decreasing the ON voltage has not been established as far as the inventors had confirmed so far.